This invention relates to improvements in the formation of optical quality surface in optical material. The term "optical material" is intended to include any material which allows a useful amount of optical energy to propagate therein or therethrough. More specifically, the term is intended to cover materials which exhibit birefringence, wherein optical energy of one polarization propagates at a different speed then optical energy of a different polarization. It also includes electro-optic materials which exhibit a change in index of refraction when a voltage is applied across the material.
A typical optical material is the class of birefringent crystals, such as, lithium niobate.
Lithium niobate and other birefringent crystals have been investigated for use in a wide range of optical systems. Such crystals are used in the fabrication of electro-optic guided wave devices. These devices are used to perform various functions om optical energy, such as switching, polarizing, combining, separating, etc. This optical energy typically carries information transmitted over optical fibers.
Typical applications of optical systems using birefringent of electro-optic crystals require efficient coupling of the optical fiber to waveguide structures formed on the crystal substrate. The optical fibers typically are formed of a central core 4-6 microns in diameter surrounded by a cladding 20-120 microns in diameter; while the waveguide structure comprises a diffused dopant channel 2-5 microns in width formed on a broad surface of the crystal.
Efficient coupling of energy from the fiber to the waveguide requires reliable, accurate horizontal, vertical and angular positioning and fixation of the ned of the fiber to an optically quality surface at the face end of the waveguide. Various techniques for accomplishing this coupling have been tried over the years with varying degrees of success.
The chemically inert nature of most birefringent crystals greatly complicates the coupling problem. A suitable etchant has not been found for lithium niobate, which removes crystal material adjacent the waveguide end face in a rapid and precise manner to form a support structure for maintaining the optical fiber in a fixed positioned abutting the end face. In particular, lithium niobate and other like materials are highly resistant to chemical or plasma etching.
Reactive ion etching (RIE) has been used to ion mill grooves (in birefringent crystals) to create microstructures. [See "Fiber-to-Waveguide Coupling Using Ion-Milled Grooves in Lithium Niobate at 1.3-.mu.m Wavelength", Optics Letters, Vol. 9, No. 10, (1984) and "Performance of Integrated Optical Frequency Shifters Pigtailed to High Benefringence Fibers Using Ion-Milled Grooves" by Andovic et al., Guided Optical Structures in the Military Environment, pp. 15/1-8, May 1986] The constraints of vacuum processing along with the limited amount of kinetic energy which can be generated for RIE have left this technique in the laboratory environment. The ablation rate achieved by reactive ion etching is of the magnitude of 3 .mu.m/hour which shows the considerable amount of time it would take to fabricate a groove of any significant depth. It is also difficult to obtain repeatable, smooth vertical walls which will yield acceptable optical surfaces. Any surface irregularity which occurs will cause optical scattering, a significant factor when trying to minimize loss. Material redeposition is also difficult to control, thus limiting the depth of the channel.
Those skilled in the art have attempted to circumvent by resorting to so-called "flip-chip" coupling using an etchable substrate fixture. In "flip-chip" coupling, a chip of birefringent crystal with a channel waveguide formed thereon is flipped over onto a readily etchable substrate, such as a silicon substrate. Preferentially etched grooves in the silicon substrate provide fiber positioning and alignment marking for accurate fiber to channel waveguide alignment. [See "Fabrication of Flip-Chip Optical Couples Between Single-Mode Fibers and LiNbO.sub.3 Channel Waveguides", Bulmer et al., Proceedings Electron Components Conference 31st, IEEE (1981)]
The substrate fixture introduces an added complexity to the problem, in that the silicon substrate is a third component which as to be thermally matched and bonded with the fiber and the crystal, per se.
Lithium niobate expands anistropically in three axis compounding the problem of bonding the Si to the LiNbO.sub.3. Currently, an optical cement is used having polymers with 10 to 20 times higher thermal expansion than LiNbO.sub.3.